Coral reefs are sea mountains of minerals which calcium carbonate predominates, along with numerous other inorganic and organic forms of calcium. Calcium is a biological glue and an inorganic building block that is ubiquitous on the planet earth. In order to build a reef, the living coral polyps require specific climatic and ecological conditions. Indeed, coral reefs are most preponderant in warm waters of the ocean which have a range of temperature from 20 degrees C to 30 degrees C, approximately.

Without sunlight the living infrastructure of organisms on the reef that use photosynthesis for nutrition cannot survive. These photosynthetic organisms (zooanthellae-algae) are quite primitive, but efficient in forming a basic nutrient source for the food chain of the reef dwellers. Some marine organisms rely heavily on the photosynthesizing organisms. For example, coral reef sponges obtain the bulk of their nutrients from these organisms and some species of jellyfish harbor photosynthesizes in their tentacles.

Of the vast number of species of coral, two distinct, basic types are recognized. These are the reef-building stony corals and the more delicate soft corals that have an inner skeleton. Not all corals from aggregations into colonies and many soft corals live among the stony corals.

The most interesting aspect of coral is their different and versatile ability to reproduce. They can reproduce by budding in an asexual manner and many polyps can form with remaining connections to its forerunner. Once a year, the corals may spawn filling areas of the reef with massive amounts of eggs and sperm (the reefs are submerged in a cloud if sperm and eggs) which attract plankton eating fish and mammals.

Oceanographers refer to several distinct types of coral reef. The coral reefs in Okinawa are associated with dormant underwater volcanoes and they tend to grow outwards to maintain their submersion in the seas. Fringing reefs are found in the Caribbean and the Great Barrier Reef and related grow along coastal plains which are raised from the seabed. Reefs can grow with amazing variations in architecture that is shaped by the geological circumstances and climate of the ocean.